I am 30 weeks pregnant, can holding your breath harm the baby?

Not likely: Babies system actually grabs & holds the oxygen from your blood better than your blood does. Although a silly practice, if you hold your breath until you pass out, you would start breathing automatically & babies system is unlikely to notice. Brief breath holding has no significant effect.

No: This will not hurt your baby.

Likely not but...: First question - why are you holding your breath? If just for swimming, it's fine. But to do it to the point of danger doesn't make sense. Best advice - breathe normally. Good luck.